SIngle shield ones are your run-of-the-mill handguards for AR-15s, the Double shield ones are the ones you see on Military M-4s.
Here's a good example; the long ones on the top are AR-15 rifle lenght, single-shield handguards, while the ones on the bottom are the shorter M4 carbine, double-shield handguards. The reason for that is that since the M4 is a select fire weapon (SAFE-SEMI-BURST/AUTO), the heat generated during BURST or AUTO fire is significantly higher than that of a SEMI ONLY rifle, therefore double shielded handguards are neccesary. Another feature of the double-shielded handguards is that the shields are staggered, also note the location of the ventilation holes - they are on a 45 degree angle instead of on the top and bottom, so the heat doesn't esacape directly to the top, blurring your sight picture. Also, the double handsguards are staggered, so they lock together ensuring not heat will leak through the sides of the handguards.
Just my .02 cents.